This commit adds the seen icon next to a message for all channels
except livechat.
It tracks received and seen messages for all members of the channel.
Icon is either 1 or 2 check(s) ('v' or 'vv').
Those are the different states of this 'seen' icon:
ICON: DESCRIPTION:
no check .................. no one has received the message.
1 check ................... some members have received the
message, no one has seen it.
2 checks (greyed-out) ..... some members have seen the message,
some or everyone has received it
2 checks (green) .......... all members have seen the message
This icon is not shown for messages preceding the last message seen
by everyone, in order to reduce visual noise.
Task-ID 1881001
Alexandre Kühn
844fb2b51e